This invention relates to a method and apparatus for manufacturing contact lenses and, in particular, to a method and apparatus for manufacturing made to order contact lenses based upon design data relating to a patient's prescription or standard design data.
Generally, contact lenses known in the art have a shape as shown in FIG. 3(a) or as shown in FIG. 3(b). A lens 300 shown in FIG. 3(a) is a minus lens used primarily to correct the visual acuity of nearsightedness. Lens 310 of FIG. 3(b) is a plus contact lens used primarily to correct the visual acuity of hypermetropia.
As seen in FIGS. 3(a), 3(b), each contact lens 300, 310 has a curved surface 301 called a base curve on the inner side thereof and another curved surface 302 called a front curve on the opposite side. The radius of curvature R.sub.1 of base curve 301 is determined to adapt to the shape of a patient's cornea, whereas the radius of curvature R.sub.2 of the front curve 302 is determined on the basis of the radius of curvature R.sub.1 of the base curve 301 and the power necessary for the patient. Minus power lens 300 has a relationship of R.sub.1 &lt;R.sub.2, whereas plus power lens 310 has a relationship of R.sub.1 &gt;R.sub.2. Where R.sub.1 =R.sub.2, the power is substantially zero D (diopter).
A curved surface bevel 303 is formed on the circumference of base curve 301. Bevel 303 generally consists of a curved surface 304 known as the secondary curve and another curved surface 305 known as the peripheral curve.
As shown in FIG. 4, a contact lens 401 is used normally in the state wherein a tear liquid 403 is interposed between contact lens 401 and a cornea 402. Since this tear liquid 403 has the important functions of surface cleaning and sterilization of cornea 402 and supplying oxygen thereto, tear liquid 403 lying between contact lens 401 and cornea 402 must be refreshed. Bevel 303 is provided to cause tear liquid 403 to smoothly flow in and out, making the vertical movement of the lens smooth. Accordingly, the shape of bevel 303 is determined based upon prescription data, such as the quantity of tear and the peripheral shape of the patient's cornea. Generally, the relationship of R.sub.1 &lt;R.sub.3 &lt;R.sub.4 exists between the radius of curvature R.sub.1 of base curve 301, the radius of curvature R.sub.3 of secondary curve 304, and the radius of curvature R.sub.4 of peripheral curve 305, but sometimes, the case of R.sub.1 &lt;R.sub.3 =R.sub.4 exists.
A curved surface 306 known as a lentic curve is formed on the circumference of front curve 302. Lentic curve 306 is provided to give a certain value to an edge thickness 307. Thus, its shape is determined based upon the radius of curvature R.sub.2 of front curve 302 and the value of a center thickness 308. Generally, the relationship between the radius of curvature R.sub.2 of front curve 302 and the radius of curvature R.sub.5 of lentic curve 306 is R.sub.2 &gt;R.sub.5 for minus power lens 300 and R.sub.2 &lt;R.sub.5 for plus power lens 310. However, the relationship R.sub.2 =R.sub.5 sometimes exists.
Clinical testing shows that edge thickness 307, which significantly influences the patient's comfort is preferably set to about 0.08 to 0.11 mm for ordinary patients, or about 0.14 to 0.15 mm for patients of advanced age whose eyelid 404 has poor tension. The edge of the contact lens is finished to a smooth curved surface to prevent damage to the cornea. Similarly, the connecting portion between base curve 301 and bevel 303 and between front curve 302 and lentic curve 306 is finished to a smooth curved surface.
Originally, to produce a contact lens, design data for a desired shape was prepared based upon a patient's prescription data or a predetermined standard specification. Then, manufacture was carried out on the basis of this design data.
Reference is now made to FIGS. 5 and 6(a) through 6(e) in which a process for manufacturing a contact lens as known in the prior art is provided. The raw material for a contact lens is a polymer or copolymer of a high molecular monomer. This material is supplied in the form of a raw columnar rod 601 as shown in FIG. 6(a). The raw material rod is cut into a button 602 (FIG. 6(a)) having a thickness larger than a finally-required thickness in a step 501. If the raw material is supplied as button 602, this first step is omitted.
A base curve 606 (FIG. 6(b)) is cut into button 602 in a step 502. As shown in FIG. 6(b), button 602 is secured to a collet chuck 604 and cut using a cutting tool 605 attached to a high-precision CNC lathe or the like resulting in a base curve 606 of desired shape.
The surface of base curve 606 is polished to provide finish to the optical surface in a step 503. As shown in FIG. 6(c), polishing is carried out utilizing a particular polishing tool 607 whose surface shape is designed in accordance with the desired shape of base curve 606. Where the base curve is to have an aspheric surface, a very small polishing pad is sometimes used.
The surface opposite the base curve is ground to form a front curve 611 and a lentic curve 612 in a step 504. As shown in FIG. 6(d), the side of the base curve 606 is blocked to a mounting tool 609 with an adhesive agent 608 and cut by the use of a cutting tool 610 attached to the high-precision CNC lathe or the like, similar to forming the base curve, to form a front curve 611 and a lentic curve 612 of desired shape. Then, polishing is again carried out using a particular polishing tool designed in accordance with the front curve shape as when polishing base curve 606 to provide an optical surface in a step 505. When a desired shape or a desired lens power cannot be obtained due to errors occurring during machining of the base curve and/or front curve, grinding is sometimes carried out during the polishing step to change the lens shape thereby correcting the error. The thus obtained lens having a desired power is generally called an uncut lens.
In a bevel grinding step 506, as shown in FIG. 6(e), an uncut lens 613 is blocked to a mounting tool 615 with an adhesive agent 614 or the like as seen in FIG. 6(e). A circumferential portion of uncut lens 613 is ground using a particular grinding tool 616 having a surface shape conforming to the patient's prescription data to form the desired shaped bevel. The circumference is cut to have a desired lens diameter. This step generally consists of forming the secondary curve, forming the peripheral curve, and cutting the circumference.
Finally, in an edging step 507, the edge and the blending portions between the individual curved surfaces are polished and ground to a finish to smooth the curved surfaces.
Since, the shape of the human eye differs from person to person, it is ideal to manufacture individual contact lenses to best adapt in shape to individual patients on the basis of the patient's prescription data. However, in view of the manufacturer's need for the efficiency of mass production to reduce the manufacturing cost and the user's demand for immediate use, a standard lens is manufactured to be used by many different patients. The present lens distribution system has adopted standard shapes adapted for a large number of patients. These standard shapes are prepared based upon clinical testing results. These standard products are previously manufactured and are stocked by optometrists and ophthalmologists and at retail stores.
One disadvantage with the prior art manufacture of the contact lens is that the manufacturing cost is extremely high. Accordingly, the above standardized system was introduced to enhance mass production efficiency and reduce the manufacturing cost by determining the specification of standard products. However, for typical standard products, the radius of curvature of the base curve varies within the range of 7.00 mm to 8.50 mm with a step of 0.05 mm, the power varies within the range of .+-.10.00 D with a step of 0.25 D, and the shape and outer diameter of the bevel, edge thickness, center thickness, etc. also vary. Therefore, the number of stock lens types which must be prepared approaches several thousand, resulting in multi-item small-quantity production. Thus, the productive efficiency is poor and it is impossible to reduce the manufacturing cost.
Further, even where a large number of standard product types are prepared, they can satisfy only about 90% of all patients, so that for the remaining 10% of the patients, it is necessary to manufacture each contact lens so as to have a specific shape adapted to each patient for each order. To satisfy each order, additional job specific tools conforming to individual shapes must be prepared, increasing the manufacturing cost and increasing the time necessary for settling the order. Another problem with manufacturing the prior art contact lenses is that it is difficult to maintain the required high degree of accuracy and the yield of manufacture is bad making it especially difficult to manufacture the contact lens having a desired power with high accuracy.
The power of the contact lens can be expressed strictly by a function of the radius of curvature R.sub.1 of the base curve, the radius of curvature R.sub.2 of the front curve and the center thickness. Conventionally, a simple method is adopted in which the difference between the surface refracting power of the front curve and the surface refracting power of the base curve is calculated. The surface refracting power d (Unit: D) of each curved surface may be expressed as EQU d=1000(n-1)/R
where n is the refractive index peculiar to a polymer or copolymer of high molecular monomer which is the raw material of the contact lens. This index varies depending on the composition of the raw material used, but it is roughly about 1.40 to 1.50. R is the radius of curvature in millimeters of the curved surface.
Assuming that the refractive index n of the contact lens raw material is 1.45 and the radius of curvature R.sub.1 of the base curve is 8.00 mm, the surface refracting power d.sub.1 of the base curve becomes EQU d.sub.1 =1000(1.45-1)/8.00 =56.25(D)
In this connection, to obtain a power of, for example, -2.00 D, the surface refracting power d.sub.2 of the front curve must be 54.25 (D) (=d.sub.2). Consequently, the front curve must be machined so as to have the following radius of curvature R.sub.2 : EQU R.sub.2 =1000(1.45-1)/54.25 =8.295(mm)
However, the machining accuracy for the base curve and front curve is limited to about .+-.0.01 mm in terms of the radius of curvature even if the best-precision CNC lathe now available is used and strict accuracy management is practiced. That is, if an error of +0.01 mm occurs during the machining of the base curve and its radius of curvature R.sub.1 becomes 8.01 mm, the resulting surface refracting power of the base curve becomes 56.18 D, resulting in an error of -0.07 D with respect to the design value. Since the allowable error of the power is .+-.0.12 D, the above error falls within the allowable range. However, if an additional -0.01 mm error occurs during the machining of the front curve and its radius of curvature R.sub.2 becomes 8.285 mm, the surface refracting power of the front curve becomes 54.32 D, and the power of the lens becomes EQU 54.32-56.18=-1.86(D)
This value shows an error of +0.14 D with respect to the design value, exceeding the allowable range of error.
In this way, the error of the power is an accumulation of the machining error arising during the machining of the base curve and the machining error arising during the machining of the front curve, so that the possibility of exceeding the allowable error is as high as about 50% to 60%, resulting in a bad yield. In the conventional lens manufacturing process, in order to overcome the foregoing drawback, at least one of the base curve and the front curve was ground to change the shape after the initial machining to correct the error of the power to fall within the allowable error limit, improving the yield. This technique is known in the art as "power change". This technique, however, inherently requires an additional step, thereby increasing the manufacturing cost. Further, since this technique is used to converge the power on an intended value, it is necessary to rely on the experience and expertise of skilled workers. Thus, no reliable accuracy can be attained.
Additionally, since the base curve and the bevel are portions which come into contact with the cornea of the patient's eye, their shapes are significant for improving the comfort of the contact lens. Additionally, they are important in preventing the movement of blood to the eye and eye inflammation resulting from contact lens use, thereby enhancing medical safety. These portions require strict accuracy in manufacture. However, because the machining of the bevel is carried out after the machining of the base curve and front curve in the conventional method shown in FIG. 5, the thickness of the contact lens raw material after machining the base curve and front curve is very small, about 0.08 to 0.15 mm, as described above. Upon the pressing of the grinding tool as shown in FIG. 6(e), its stress causes a deformation of the contact lens raw material, thereby preventing accurate formation of the bevel. Since this forming step has been previously performed based upon the degree of deformation determined from the long-term experience and expertise of skilled workers, this manufacturing method provides a very low productivity and a very bad yield.
Further, the polishing of the base curve, front curve, etc. has been previously carried out using job specific shaped polishing tools to maintain a demanded accuracy. Therefore, separate polishing tools must be prepared which are individually adapted in shape to the base curve, secondary curve, peripheral curve, front curve and lentic curve, thereby increasing the number of different polishing tools required. This increases the manufacturing cost, and requires changing the polishing tools upon the changing of the lens type, thereby lowering the productivity.
According to the conventional method, the base curve is machined and polished with the side of the front curve secured as shown in FIG. 6(b). The front curve and lentic curve are machined and polished with the side of the base curve blocked as shown in FIG. 6(d). Then, the bevel is machined with the side of the front curve again secured as shown in FIG. 6(e). Therefore, there are three steps for securing the contact lens raw material. Consequently, since the work of securing the lens also involves an error in relation to the securing position, this error is accumulated due to the repetition of the securing step three times. As a result, the possibility of the shape defectiveness arising from this securing position error, such as prism defectiveness and eccentric defectiveness, exceeding the allowable limit, lowers the yield. Since the shape of the base curve differs depending on the kind of lens, the securing position tends to shift when the side of the base curve is secured as shown in FIG. 6(d). As a result, the machining position of the front curve deviates, and the optical axis of the front curve comes out of agreement with that of the base curve; resulting in prism defectiveness and/or center thickness error which exceeds the allowable limit. Similarly, positional shift also tends to occur when the side of the front curve is secured as shown in FIG. 6(e). As a result, an eccentric defect arises, or the position of the bevel deviates eccentrically, thereby lowering the accuracy in shape of the bevel.
Accordingly, it is desired to provide an improved method and apparatus for manufacturing a contact lens which overcomes the disadvantages of the prior art devices described above.